I don't think the banana is a good idea. One fellow on here added it to mead with bad results:
If you wanted more banana flavor you could probably add some banana extract to the primary, or even before bottling. I would be careful though, extracts can be really powerful.
I think the banana flavor you might be looking for will result from stressing the yeast to produce esters, or just from using a traditional Belgian Wit yeast. A higher fermentation temperature ~ 75 F will help.